1. Field of the Invention
This invention relates to a sensor rotor unit and a bearing assembly, and in particular to a sensor rotor unit which cooperates with a sensor to detect the number of rotations of a rotary member made integral with a rotating wheel.
2. Related Background Art
In various industrial machines, it sometimes becomes necessary to use a bearing assembly including inner and outer races rotating relative to each other through rolling members in order to support a rotary member and moreover, to detect the number of rotations of the rotary member. For example, the anti-skid brake system in a vehicle has such requirements. This system detects the number of rotations of a wheel and momentarily releases the brake when the wheel is about to be locked, thereby preventing the locking of the wheel. For the detection of the number of rotations, use is suitably be made of a sensor rotor fixed to the rotating wheel and an electro-magnetic pick-up type sensor.
Heretofore, an annular member of a magnetic material having a concavo-convex gear portion formed on the outer peripheral surface thereof has been adopted as the sensor rotor, and it has been the practice to fit this annular member to the outer peripheral surface of the outer race. To form the gear portion, it is necessary to form an annular blank by a broaching process or machining by the use of a gear cutting machine, or by sintering. However, the broaching process or the like has resulted in a high working cost, and sintering has led to a problem in respect of strength as well as a problem that rust prevention is difficult.
So, to solve such problems, the applicant devised the invention described in Japanese Utility Model Application Laid-open No. 63-87514. In this application, a plate-like member is used as a blank forming a pulsar gear, and a window or a hole is formed therein by punching. If this is done, the abovenoted problems are eliminated, but a problem regarding rust prevention remains.
In practice, the bearing assembly is sometimes used under conditions in which it is exposed to rainwater. In order to prevent rust from being created due to rainwater, it is desirable to subject the sensor rotor to rust preventing treatment. However, even if the sensor rotor is subjected to the rust preventing treatment, if the rotating wheel of the bearing is not subjected to the rust preventing treatment, rust created on the bearing due to rainwater is propagated to the sensor rotor (extraneous rust). Thus, the sensor rotor may be rusted, whereby the wave form is disturbed and accurate detection is hampered.
It is not a simple matter to subject the rotating wheel of the bearing to the rust preventing treatment. The reason is that the rotating wheel is usually already subjected to heat treatment for hardening, and if it is thereafter subjected to the rust preventing treatment, the hardness thereof is reduced by high heat applied thereto at that time.